1. Field of the Invention
The present invention relates generally to frame communication apparatus and method in a broadband communication system. More particularly, the present invention relates to an apparatus and method for reducing a frame overhead in a wireless communication system using an Orthogonal Frequency Division Multiple Access (OFDMA) scheme.
2. Description of the Related Art
Many wireless communication technologies have been proposed for high speed mobile communication. Among the technologies, Orthogonal Frequency Division Multiplexing (OFDM) is considered as the most potential next generation wireless communication technology. For example, Institute of Electrical and Electronics Engineers (IEEE) 802.16 based Wireless Metropolitan Area Network (WMAN) employs the OFDM technology as a standard specification.
FIG. 1 is a diagram of a frame structure in a conventional OFDMA system.
Referring to FIG. 1, data transmission units of conceptual frequency and time domains in the OFDMA standard are subchannel and symbol, respectively. A minimum data unit that can be transmitted to one user (terminal) consists of one subchannel and one symbol. A vertical axis represents index of a subchannel that is a frequency resource allocation unit. One frame consists of (L+1) number of subchannels from sth subchannel to (s+L)th subchannel. A horizontal axis represents index of OFDM symbol that is a time resource allocation unit. One frame consists of (M+1) number of downlink OFDM symbols, from kth downlink OFDM to (k+M)th downlink OFDM symbol, and N number of uplink OFDM symbols, from (k+M+1)th uplink OFDM symbol to (k+M+N)th uplink OFDM symbol. A Transmit/receive Transition Gap (TTG) exists between the downlink and the uplink. The TTG is a time guard region.
In the OFDMA frame, the downlink interval includes a preamble, a frame control header (FCH), a downlink map (DL-MAP), an uplink map (UL-MAP), and downlink bursts (DL-bursts). The uplink interval includes uplink bursts (UL-bursts). The preamble is used for providing users with time synchronization, frequency synchronization, and cell information. The FCH contains information for decoding the DL-MAP. The DL-MAP contains information indicating which user data the DL-bursts transmitted from a base station are, and which region the user data are located in within the frame. The UL-MAP contains information on the UL-bursts transmitted from the user terminals.
One burst includes at least one subchannel and one symbol. Order of the symbol is physically identical to that of time, so that kth symbol, (k+1)th symbol, . . . , and (k+M+N)th symbol are arranged in sequence. On the other hand, sth subchannel and (s+1)th subchannel may or may not be physically adjacent to each other. Because of characteristics of the OFDM technology, the signal has a frequency selective characteristic when passing through a radio channel. Therefore, the subchannels are constructed with subcarriers that are not physically adjacent to each other.
The process of mapping the actual physical subcarriers to the subchannels to construct one logical subchannel is referred to as a subchannel allocation. IEEE 802.16 OFDMA standard uses several subchannel allocation schemes, including a diversity subchannel allocation scheme and an Adaptive Modulation and Coding (AMC) subchannel allocation scheme. The diversity subchannel allocation scheme also includes a Full Usage of Subcarrier (FUSC) and a Partial Usage of Subcarrier (PUSC). The diversity subchannel allocation scheme can well cope with a radio channel having frequency selective characteristic by scattering the physical subcarriers of the logical subchannels. Meanwhile, the AMC subchannel allocation scheme constructs the subchannel with the subcarriers physically adjacent to one another. The AMC subchannel allocation scheme can determine the subchannels having good channel situations among the frequency selective subchannels and increase the throughput by changing the subchannel modulation and the channel coding.
As described above, the IEEE 802.16 OFDMA system adaptively copes with the radio channel environment by using various subchannel allocation schemes, and maximizes the degree of freedom in the frame structure by constructing the uplink and downlink bursts of the frame through the allocation of at least one subchannel and at least one symbol.
However, as the degree of freedom increases, control information to be transmitted also increases. Therefore, when a plurality of user data are mixed within the frame, control information notified to the user through the DL-MAP and the UL-MAP serves as the overhead. As one example, in a 2084-Fast Fourier Transform (FFT) mode having a bandwidth (BW) of 10 MHz, at least 43 bits are required to inform one user of the data location in the frame. The 43 bits include “CID (16 bits)+starting point of data (14 bits)+data size (13 bits)”. If adding the subchannel allocation scheme and other necessary control information, control information that must be actually transmitted will greatly increase. When there are a lot of users, a small amount of data can be merely transmitted.
In addition, the data packets of the DL-burst are several users' packets using the same Modulation and Coding Scheme (MCS) level. The users' own data location cannot be accurately known only by the location of the DL-burst notified from the DL-MAP. Therefore, as illustrated in FIG. 2, CID (16 bits) identical to that used in the MAP channel to discern the user's own data is included in a header of user data (packet). Because the same information (CID) is recorded in the header of each data packet as well as the MAP channel, the data throughput decreases.
Accordingly, there is a need for an improved apparatus and method for increasing data throughput in a wireless communication system.